Finding safe, reliable and inexpensive means for making noncombustible products is one of the most important problems facing many industries today. Regulations relating to the flammability of building products, textiles, appliances and the like are becoming much more stringent, and will continue to be more stringent as the public is made more aware of the hazards presented by combustible materials. The hardboard industry is no exception to this trend, and the use of hardboard paneling is currently being restricted because of its inherent flammability.
The standard test for the flammability of building products is the ASTM E84 (Steiner tunnel) test. By this test, materials are rated for: (1) flamespread index (FSI) (2) fuel contribution, and (3) smoke generation. All materials are rated relative to asbestos cement board (0 on all indices) and red oak (100 on all indices). Hardboard typically receives a flamespread index of 175-200 (Class III), a fuel contributed value of 150, and a smoke index of 400-600. In contrast, a low-hazard material such as gypsum board typically receives a flamespread of 10-15 (Class I), a fuel contributed value of 15-30 and a smoke index of 0. While all three indices are important, flamespread and smoke are the major concern. Class I products have indices of 0-25, Class II products range from 26-75 and Class III materials have indices between 76 and 200.
In testing various materials in the laboratory, a 2 feet standardized lab tunnel, commonly known as a Monsanto tunnel (J. Paint Technology, Vol. 39, No. 511, page 494, 1967, and Vol. 46, No. 591, pages 62-69, 1974) is used. This tunnel consists basically of a 24 .times. 4 inch angle iron frame inclined at 28.degree. from the horizontal into which the board sample (231/2 .times. 33/4 .times. 0.2-0.3 inch) is placed. A gas burner (Fisher Scientific No. 3-902) is mounted so that the burner flame impinges the lower end of the board. The maximum distance that the burning flame tip moves up the inclined board under controlled gas flow in four minutes is reported. A reliable correlation has been found between the Monsanto 2 feet tunnel and the 25 feet Steiner tunnel. If the longest flame length in the Monsanto tunnel during the 4 minute test is 12-13 inch the panel would attain a Class I rating in the E84 test. Flamelengths up to 17-18 inch correspond to Class II materials. No smoke or fuel measurements have been made on the Monsanto tunnel. The Steiner tunnel uses both time and distance to determine flamespread indices and due to the method of calculation many treatments will reduce the flamespread index of hardboard to the 90-110 range. It is much more difficult, however, to reduce the flamespread index from 100 to 75 since the flammability must almost be reduced by an additional one-half to move these last 25 points into the Class II range. The synergistic mixture of alumina trihydrate and borate readily reduces the flamespread index of hardboard to the Class I or Class II level.
Several approaches have been taken to reduce the flame-spread of hardboard in previous years when flammability was not quite as crucial a concern. Physical characteristics such as caliper, specific gravity, and embossing were examined for their effects on flamespread and it was found that denser, thicker panels received the lowest flamespreads, while embossing had little effect. Asbestos and metal overlays were tried as a means of reducing flamespread, but with little success. It became evident that flamespread reduction was more than just a surface phenomenon, involving instead the characteristics of the entire board. With this in mind, a variety of commonly accepted chemical treatments were attempted, but none could sufficiently reduce the flamespread to an acceptable level without adversely affecting moisture sensitivity and other board properties. Severe production problems were also common to these treatments.
More recently attention has been diverted from chemical treatments to the concept of fuel dilution. As the term suggests this involves substituting noncombustible (generally inorganic) materials for wood fiber until the desired flamespread reduction is achieved. Little work had been done previously with this concept since it is normally considered an inefficient mechanism, requiring too much expensive substitute material. Dilution levels of 65-80% are not unusual to attain select levels of fire retardance. In arriving at the present invention, an extensive evaluation of various diluents was undertaken in the fabrication of hardboard. Materials such as fly ash, cement, vermiculite ore, slag, and mineral wool were examined. Mineral wool was selected as the most likely candidate for investigation since it was fibrous in nature and low in cost and an extensive evaluation of fuel dilution in hardboard followed. As suspected, very high dilution levels on the order of 75% (75 parts mineral wool -- 25 parts wood fiber) were required to achieve a Class II flamespread rating. At such low wood fiber levels the physical properties of the board were very poor, so an extensive amount of work was done to bring these properties up to an acceptable level. Eventually, acceptable interior and exterior Class II hardboard formulations were developed in the laboratory but attempts to produce such formulations in the plant met with little success because the mineral wool proved too fragile for existing equipment and quickly broke down into fines. Low levels were added successfully, but at high levels the wetlap became muddy and unhandleable. Dry process formulations were developed in the laboratory but were not run at the manufacturing plants.
Work with the mineral wool indicated that if substitution levels could be reduced to 50% or less the chances for success with fuel dilution on existing plant equipment would be much better. Studies showed that there was little difference between inert fuel diluents except as flamespread was affected by inadequate retention and distribution. What was needed therefore was a low-cost fuel diluent that would provide an active contribution to flamespread reduction of the remaining combustible fibers in addition to its passive roll as a diluent.
One such diluent meeting these requirements was alumina trihydrate, Al.sub.2 O.sub.3.3H.sub.2 O. This white powdery material is approximately 35% by weight water and exhibits a considerable endotherm in the same temperature range at which hardboard begins to undergo severe pyrolysis. Little or no water of hydration is lost during hot pressing. Thus, this material could be used as an active fuel diluent and on the basis of its thermal data should be more effective than mineral wool. This was subsequently shown to be true as 40-45% alumina trihydrate was as effective as 65-75% mineral wool. This research work was then moved to the plant trial stage.
During the initial alumina trihydrate plant trials boards with 40-45% diluent were targeted but these levels were not reached because of wetlap handling problems. No Class II hardboard was made since 32-34% alumina trihydrate was the maximum level attained. Products with this level attained low Class III flamespread ratings in the 25 feet tunnel. As the program progressed into its later stages and operating personnel became more familiar with running this diluent, the 40-45% levels were reached; but it was believed necessary in the initial stages to limit the alumina level to 32%, so a means of further increasing its effectiveness was sought. This led to examination of mixtures of chemicals and fuel diluents in an effort to combine the expertise in each of these areas into one cause.